Capacitor-to-voltage converters are needed in applications which use a capacitive sensor element. These applications include tire pressure measurement systems, acceleration measurement systems or others. These systems usually include an external capacitive sensor which is outside an integrated circuit that includes readout circuitry. The capacitance of the capacitive sensor changes, for example, with pressure. For further processing, such as digitizing with an analog-to-digital converter, this capacitance is changed into a capacitance dependent voltage by a capacitance-to-voltage converter.
Usually, the capacitive sensor element and the readout circuitry are assembled together on a printed circuit board. One requirement is that the input pins of the readout circuitry must withstand electrostatic discharges, for example, with voltages up to 2 kV according to the human body model. This protection is achieved by ESD (electrostatic discharge) protection circuits. Unfortunately, these ESD protection circuits cause substrate leakage currents which increase with temperature. In an automotive environment, the temperature ranges up to 125° Celsius. At this high temperature, conventional ESD protection circuits lead to a leakage current of typically 50 nA. This leakage current falsifies the readout of the capacitance; thus, falsifying the voltage to be digitized. Especially, the temperature dependency of the leakage causes problems.
One approach to increase the precision of capacitance measurement is to minimize the leakage current. Therefore, the use of special ESD protection circuits is necessary, but this leads to an increase of costs of the integrated readout circuitry and reduced ESD protection performance. Furthermore, leakage currents may also occur independent of the ESD protection circuit due to dirt or humidity on the printed circuit board or due to assembly problems, as well as due to chip internal resistive elements.
Therefore, there is a need for a method of capacitance-to-voltage conversion which cancels out the effect of any leakage currents, even when using standard ESD protection circuits.